Flow

Note about flow words: flow words can only be used in
definitions. In the INTERPRET loop, they don't have the desired
effect because each word from the input stream is executed
immediately. In this context, branching doesn't work.

(br)  --   Branches by the number specified in the 2 following
           bytes. Can be negative.
(?br) f -- Branch if f is false.
(     --   *I* Comment. Ignore rest of line until ")" is read.
[     --   Begin interpretative mode. In a definition, words
           between here and "]" will be executed instead of
           compiled.
]     --   End interpretative mode.
ABORT --   Resets PS and RS and returns to interpreter. (cont.)